The circuit for the drain of Tr1 simply isn't right: amongst other things, the wiper of VR1 is 'hot' as far as R.F. is concerned. If you are finding that the reaction is too harsh and kicks in suddenly, increase the distance between L1 and L2. That will reduce the mutual inductance and thus reduce the degree of feedback. It's also worth bearing in mind that the gm of the '3819 can vary widely from one example to the next - as I found out many years ago when I first started using them for audio and V.H.F. pre-amplifiers.
For circuit development work, especially R.F. circuitry, S-DeC is the last thing you want to use.
Here's what I do: take two pieces of double-sided copper laminate board, typically one 6" x 5", the other 6" x 3". Solder the two 6" sides together at right angles. The narrower piece becomes the 'front panel'; the wider piece becomes the 'chassis'. Drill a number of holes in the front panel first - you can use these holes to mount things like pots., variable caps, switches, etc. For the chassis, simply use point-to-point connections with the chassis as the 0v./ earth point. You can usually arrange the component layout such that those earthing points will give sufficient rigidity to the associated components. Sometimes the use of conventional tag-strip helps: again, the earthing tags of those strips can be soldered direct to the chassis. For critical circuits where unwanted feedback may be a problem, simply solder in additional pieces of copper board to act as screens.
When you're happy with the final cct. and go to 'production, you can strip off all those components and then the 'test chassis' is ready for the next development project.
Al.
For circuit development work, especially R.F. circuitry, S-DeC is the last thing you want to use.
Here's what I do: take two pieces of double-sided copper laminate board, typically one 6" x 5", the other 6" x 3". Solder the two 6" sides together at right angles. The narrower piece becomes the 'front panel'; the wider piece becomes the 'chassis'. Drill a number of holes in the front panel first - you can use these holes to mount things like pots., variable caps, switches, etc. For the chassis, simply use point-to-point connections with the chassis as the 0v./ earth point. You can usually arrange the component layout such that those earthing points will give sufficient rigidity to the associated components. Sometimes the use of conventional tag-strip helps: again, the earthing tags of those strips can be soldered direct to the chassis. For critical circuits where unwanted feedback may be a problem, simply solder in additional pieces of copper board to act as screens.When you're happy with the final cct. and go to 'production, you can strip off all those components and then the 'test chassis' is ready for the next development project.
Al.
